Aerial cable with external radio signal reflection correction



April 29', 1952 F. BLOCH ,8

AERIAL CABLE WITH EXTERNAL RADIO SIGNAL REFLECTION CORRECTION Filed Dec.10? 1945 I v INVENTOR FELIX BLOCH BY 4 V a-m. 9M.

ATTORNEY' Eatented Apr. 29, 1952 AERIAL CABLE WITH EXTERNAL RADIO SIGNALREFLECTION CORRECTION Felix Bloch, Palo Alto, Calif., assignor to theUnited States of America as represented by the Secretary of WarApplication December 10, 1945, Serial No. 634,091

This invention relates generally to electrical apparatus and moreparticularly to cables for towing targets for radio object locatingapparatus.

One type of cable that has been used previously for towing targets forradio object locating apparatus is inch armored steel cable. This cablehas the disadvantage of reflecting radio frequency energy from thenearby radio object cating apparatus. This causes undesirable echoes toappear at the radio object locating apparatus and if the set is anautomatic tracking device the tracking will be unreliable.

It is an object of this invention, therefore, to provide a cable fortowing radar targets which will have a relatively low radio frequencyreflectivity.

It is a further object of this invention to provide a cable with lowreflectivity and with mechanical strength and flexibility equal to thatof cables with similar ratio of weight to length in cables previouslyused.

Other objects, features, and advantages of this invention will suggestthemselves to those skilled in the art and will become apparent from thefollowing description of the invention taken in connection with theaccompanying drawing in which the sole figure is a perspective view of asection of cable constituting one embodiment of the invention.

Referring now to the drawing, the inner steel cable 22 is covered withan elastic, flexible, rubber-like insulating material [2. Over theinsulating material I2, and spaced longitudinally at intervals aresleeves of conducting material i3 such as aluminum foil. Over theinsulating material I2 and conducting material sleeves l3, there iswrapped spirally an armor material I4 such as steel.

The operation of the cable described above can best be understood byconsidering the reason why an ordinary cable has a high reflectivitywhen irradiated with radio frequency energy of normal incidence. In thiscase, a uniform current is caused to flow along the cable, and theresulting secondary or reflected waves will have eifectively overordinary ranges of radio object locating frequencies the same phase overthe whole length of the cable. This results in a strong return signal.

My above-described type of cable prevents a uniform current flow,because the self-inductance per unit length of cable is no longerconstant over the length of cable, but comprises a group of difierentlow values originating in the 2 Claims. (Cl. 17844) shorted andunshorted sections, and in splitphase relation, since the conductingmaterial sleeves l3 act to short out a number of turns of the armormaterial l4. Consequently, the secondary waves originating fromdifferent regions of the cable will no longer have the same amplitude,nor necessarily the same phase. The distance between sleeves [3 has beenadjusted in such a way as to obtain the best possible destructiveinterference of these secondary or reflected waves. It is for thisreason that the spacing between the sleeves I3 is a critical dimensionof the above-described cable.

In practice, the operation of the improved cable is similar to theordinary A; inch armored steel cable with the exceptions that the armorI4 is separated from the inner steel cable II by a thin layer ofinsulating material I2, and that certain turns of the spiral armor [4are shorted out by the metal sleeves I3. With this particular cable andthe particular frequency of the transmitter with which it was tested, itwas found that for best operation the conducting sleeves I3 should bespaced one inch apart and that each sleeve I3 be inch in length. Thesleeves [3 were fastened to the insulating material 12 with a substancesuch as glue. The most critical dimension was found to be the spacingbetween sleeves l3. Tolerance of this dimension should be held toapproximately 3 2 inch.

It was found that the reflectivity of this type of cable was reducedwhen compared with the conventional cable at a frequency in theneighborhood of several thousand megacycles.

While there has been described what is at present considered thepreferred embodiment of this invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departin from the scope of the invention as set forth inthe appended claims.

What is claimed is:

1. In an aerial cable for use in a zone of transmission of radioobject-locating signals, an inner cable constructed of a continuousconducting part, an insulating material at least partially covering saidinner cable, sleeves constructed of a conducting material overlying saidinsulating material, said sleeves being spaced at intervals along thelength of said cable, and armor material spirally wound around saidinsulating material and said sleeves with interstitial space between theconvolutions, the sleeve lengths and spaces between the sleeves beingproportioned in 3 a definite relation to a given band of external radiosignal frequencies incident externally on the cable.

2. The article of claim 1 in which the said spaces between the sleevesexceed in longitudinal 5 extent the lengths of the sleeves.

FELIX BLOCH.

REFERENCES CITED Number The following references are of. record, in the1 Number file of this patent:

4 UNITED STATES PATENTS Name Date Emanueli Mar. 13, 1928 Wrape Jan. 21,1930 Droste Aug. 29, 1933 Wentz Apr. 28, 1936 Roosenstein Apr. 29, 1947FOREIGN PATENTS Country Date Great Britain Dec. 27, 1922

